Recent developments in silencing the expression of specific genes, through RNA interference pathways, have led to an increased demand for synthetic RNA sequences and have created a pressing need for rapid and efficient methods for RNA synthesis. Some of the requirements for the production of RNA sequences in sufficient quantity and purity for pharmaceutical applications are those related to the design and implementation of hydroxyl protecting groups, which have historically been demonstrated to be of critical importance in RNA synthesis. Furthermore, chemical modifications of the hydroxyl function of ribonucleosides have been extensively investigated in order to identify the biophysical and biochemical parameters that are necessary for effective and lasting RNA interference-mediated gene silencing activities. For example, 2′-hydroxy modifications impart high binding affinity to RNA sequences, increased lipophilicity, enhanced chemical stability and nuclease resistance. The 2′-hydroxy group of ribonucleosides is also an attractive function for ligand attachment; there are numerous examples of ribonucleoside 2′-conjugates that have been reported in addition to therapeutic and diagnostic applications.
Although 2′-O-alkylation of ribonucleosides with either the ligand itself or with a functional group amenable to indirect ligand attachment has often been employed in the synthesis of ribonucleoside 2′-conjugates, this method is generally limited by low yields and lack of regioselectivity. Attempts have been made to prepare aminooxy-modified oligonucleotide synthetic intermediates; see for example, U.S. Pat. No. 6,194,598, which reports the synthesis of 2′-O-aminooxyethyl derivatives. The 2′-O-aminooxyethyl derivatives, however, have certain drawbacks such as the protection offered by this derivative to 2′-OH group is more permanent than desirable.
The foregoing shows that there exists an unmet need for O-protected derivatives of nucleosides that are easy to prepare and that the protecting group is easily removed. There also exists an unmet need for a method of synthesis of oligonucleotides based on such O-protected derivatives.